1. Field of the Invention
The present invention relates to an optometric apparatus and a lens power determination method to perform a subjective eye examination by prompting a subject to view test symbols displayed on a display means using one of the right and left eyes at a time. More particularly, the present invention relates to an optometric apparatus and a lens power determination method which are suitable for eye examinations performed to determine lens powers, e.g., in order to sell eyeglasses or contact lenses over the Internet.
2. Description of the Related Art
Conventionally, to determine the refractive power of eyeglasses or contact lenses, a conventional method used an auto-refractometer to objectively determine the refractive coefficient of the eyeball and then the subject actually wore ready-made corrective lenses in order to test the subject's visual acuity.
However, such an auto-refractometer is very expensive and requires expert knowledge to operate. Additionally, to actually wear corrective lenses when testing visual acuity, the subject had to visit an ophthalmologist or to go to an eyeglass shop for an eye examination, where various types of corrective lenses were available. Thus, it was difficult to order eyeglasses or contact lenses at shops or at home where no such facilities were available.
Recently, with the advancement of computers and network technologies, a system is being developed which allows the user to perform a subjective eye examination at shops or at home where no facilities, such as an auto-refractometer or corrective lenses are available (e.g., A remote visual acuity determination system disclosed in Japanese Patent Laid-Open Publication No. 2001-286442).
Such a conventional system displays test symbols for determining visual acuity or “Landoldt rings”, as shown in FIG. 21, to the subject in various sizes on a computer screen. The system then allows the subject to select the smallest viewable test symbol for each of the right and left eyes, thereby determining the subject's visual acuity. In addition, for subjects having astigmatism, the system displays a rotating Landoldt ring for the subject to determine the orientation in which the opening or break appears closed. The system also displays a test symbol, shown in FIG. 22, for determining an astigmatic axis on a computer screen, and the subject selects the orientation in which the subject is viewed with the highest contrast (or most clearly or most sharpest) for each of the right and left eyes, thereby determining the subject's astigmatic axis. Then, the visual acuity is determined based on the test symbols for determining visual acuity at the determined astigmatic axis and in an orientation orthogonal thereto.
Alternatively, the system may be used by an unlimited number of people via the Internet, so some of the subjects may have hyperopia. However, the conventional optometric device simply provided test symbols in various sizes for the subject to select the smallest viewable test symbol. Thus, the difference between myopia and hyperopia cannot be distinguished, which may provide misleading eye examination results.
In particular, since the eye examination performed on a computer screen requires the entry of results of viewing by the subject using a mouse or the like, the distance between the subject's eye and the test symbol is restricted within a specific range. Thus, it is difficult to distinguish between hyperopia and myopia using only the test symbols for determining visual acuity.
Furthermore, although some subjects with astigmatism may have mixed astigmatism which causes one of the major and minor axes to exhibit myopia and the other to exhibit hyperopia, the conventional optometric device cannot be applied to these subjects.